Apparatus and method for confocal microscopy using dispersed structured illumination

ABSTRACT

Methods and apparatus are presented for confocal microscopy using dispersed structured illumination. In certain embodiments the apparatus also comprises an optical coherence tomography (OCT) system, and OCT images acquired from two or more regions of a sample are registered using a corresponding set of two or more larger area images acquired with the confocal microscopy system. In preferred embodiments the apparatus is suitable for analysing the retina of an eye. The confocal microscopy system can be operated in a purely intensity mode or in a coherent mode. In other embodiments a confocal microscopy system using dispersed structured illumination is utilised for surface metrology.

RELATED APPLICATIONS

The present application claims priority from Australian ProvisionalPatent Application No. 2016902602 entitled ‘Apparatus and method forconfocal microscopy using dispersed structured illumination’ filed on 1Jul. 2016, the contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to apparatus and methods for confocalmicroscopy using dispersed structured illumination, in particular forthe registration of co-acquired optical coherence tomography (OCT)images. However it will be appreciated that the invention is not limitedto this particular field of use.

BACKGROUND OF THE INVENTION

Any discussion of the prior art throughout the specification should inno way be considered as an admission that such prior art is widely knownor forms part of the common general knowledge in the field.

Optical coherence tomography (OCT) is a widely used interferometrictechnique for studying biological samples including in vivo tissue suchas the human eye, with lateral and depth resolution, using informationcontained within the amplitude and phase of reflected or scatteredlight. OCT systems generally utilise a Michelson interferometerconfiguration, with two main approaches being employed: time domain OCTand spectral domain OCT.

In time domain OCT coherence properties of a partially coherent sourcesuch as a superluminescent light emitting diode (SLED) with a coherencelength of several microns are utilised by interfering light reflectedfrom a sample with a reference beam provided by the same source, butwith a time-varying path length. At a specific depth in the samplecorresponding to the path length delay in the reference arm, aninterference envelope of fringes will be detected in the combinedback-reflected signal, allowing the reflection profile in the depthdimension to be reconstructed. Commonly this is done for only a singlesample point at a time, and the corresponding scan of depth is known asan ‘A-scan’.

Instead of scanning a delay line, spectral domain OCT techniques analysethe reflected light by interfering it with a reference beam, either as atime-varying function of wavelength (swept source OCT) or by dispersingthe different wavelengths with a grating or other spectral demultiplexerand detecting them simultaneously along a detector array. The spectraldomain information is the Fourier transform of the spatial (depth)reflection profile, so the spatial profile can be recovered by a FastFourier Transform (FFT). Generally speaking, spectral domain OCT systemsare preferred over time domain OCT systems because they have a ˜20 to 30dB sensitivity advantage.

OCT techniques can be adapted to provide a laterally resolved ‘B-scan’by scanning the sample beam relative to the sample in one axis, or a‘C-scan’ by scanning in two axes. Faster acquisition is generallydesirable irrespective of the type of scan, especially for reducingmotion-induced artefacts with in vivo samples, and has been greatlyimproved over the previous 20 to 25 years by advances in several fieldsincluding faster swept source scanning rates and photodetector arrayreadout speeds. However a fundamental limitation with scanning spotschemes, especially for in vivo applications, is presented by lasersafety regulations: reducing dwell time to increase scanning speedwithout being able to increase the applied power will inevitably degradethe signal to noise ratio.

Consequently there has also been research into ‘parallelised’ OCTsystems in which an extended sample area is probed with lateralresolution, or an array of sample spots probed simultaneously. It isrelatively straightforward to parallelise time domain OCT, e.g. byutilising a CCD camera and imaging optics as described in U.S. Pat. No.5,465,147 entitled ‘Method and apparatus for acquiring images using aCCD detector array and no transverse scanner’. This provides a twodimensional (2-D) en face image, with depth resolution provided byscanning the reference mirror as usual in time domain OCT.

Swept source spectral domain OCT can be parallelised in similar fashion,as described in Bonin et al ‘In vivo Fourier-domain full-field OCT ofthe human retina with 1.5 million A-lines/s’, Optics Letters 35(20),3432-3434 (2010). However because each frame corresponds to a singlewavelength, the acquisition time for each A-scan is equal to the frameperiod times the number of k-points (wavelength samples) acquired. Evenfor very high speed cameras with frame rates of 100s of kHz, this canlead to A-scan acquisition times of many ms which can lead to motionartefacts especially with in vivo samples. Published PCT patentapplication No WO 2016/094940 A1, entitled ‘Multichannel opticalreceivers’, discloses an alternative parallelised swept source OCTscheme that enables faster acquisition. In one particular implementationa plurality of spots on a sample are illuminated simultaneously and thereflected or scattered signal light mixed with a reference beam to forma plurality of interferograms with unique carrier frequencies.

Spectrometer-based spectral domain OCT is somewhat more difficult toparallelise because of the necessity to disperse wavelength across manypixels of a 2-D sensor array. In a configuration described in publishedUS patent application No 2014/0028974 A1 entitled ‘Line-fieldholoscopy’, cylindrical lenses are used to produce a line illuminationon a sample and on a reference mirror. Dispersion of the combined returnsample and reference beams along one axis of a 2-D sensor array enablessingle shot B-scan acquisition. However for full three-dimensional(C-scan) imaging the illuminated line needs to be scanned in theorthogonal direction and the 2-D sensor array read out repeatedly, andit is generally difficult to retain phase coherence between the repeatedlinear scans.

Single shot C-scan acquisition can be achieved if a 2-D sample area isilluminated and the combined returning sample and reference wavefrontssampled in the two lateral dimensions, e.g. with a 2-D lenslet array,and the resulting sampling points dispersed onto separate sets of pixelsof a 2-D sensor array. The effect of this general scheme is to squeezedata from three spatial dimensions, equivalent to two lateral dimensionsand one spectral dimension, onto a 2-D sensor array. A mapping ofdispersed sampling points onto separate sets of pixels can be ensured byappropriate positioning of the sampling points with respect to thewavelength dispersive element. U.S. Pat. No. 9,243,888 entitled ‘Imagemapped optical coherence tomography’ discloses an alternative approachin which an ‘image mapper’ having a number of differently angled facetsreflects light from different portions of an image onto different areasof a dispersive element and thence onto separate sets of pixels of a 2-Dsensor array.

Configurations for applying a lenslet-based sampling technique to singleshot acquisition of images from small volumes of order 100 μm×100μm×1000 μm from retinal and other samples are disclosed in published USpatent application No 2016/0345820 A1 entitled ‘High resolution 3-Dspectral domain optical imaging apparatus and method’ and Anderson et al‘3D-spectral domain computational imaging’, Proc SPIE 9697 (8 Mar. 2016)http://dx.doi.org/10.1117/12.2214801. The combined returning wavefrontsare sampled with a rectilinear lenslet array angled with respect to thedispersive axis of a dispersive element, with the sampling being ineither the Fourier plane, i.e. the far field, providing a form of‘holoscopy’, i.e. holographic OCT, or the image plane, i.e. the nearfield. In either case the spatial resolution depends largely on the NAof the objective lens, and may for example be around 3 μm. Illuminatedareas on the sample are preferably kept relatively small, of order 100μm×100 μm, to reduce the impact of multiple scattering and also becauseof the limited number of sampling points offered by commerciallyavailable lenslet arrays. Images of multiple adjacent or overlappingvolumes can in principle be acquired and stitched together to imagelarger sample volumes, but the total acquisition speed is limited by theframe rate of the 2-D sensor array. Although acquisition of individualvolumes can be fast, of order 0.1 ms, for reasonably cost effectiveelectronics the frame rate currently limits the acquisition rate to afew 100 Hz. Since eye motion can be significant on the ms time scale,improved registration techniques are required to allow a larger area ofa retina to be acquired by stitching together frames without loss ofregistration. Improved registration techniques would also beadvantageous to provide a clinical user with a broader view of a retinaduring setup, while identifying areas of interest. While a separateoptical image of the retina could be obtained and multiplexed into anOCT image, this would provide inferior resolution detail and theseparate imaging systems are not tightly integrated.

Unless the context clearly requires otherwise, throughout thedescription and the claims the words ‘comprising’, ‘comprises’ and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense. That is, they are to be construed in thesense of ‘including, but not limited to’.

OBJECT OF THE INVENTION

It is an object of the present invention to overcome or ameliorate atleast one of the limitations of the prior art, or to provide a usefulalternative. It is an object of the present invention in a preferredform to provide apparatus and methods for acquiring optical coherencetomography images of extended sample volumes by stitching together twoor more images without loss of registration. It is another object of thepresent invention in a preferred form to provide apparatus and methodsfor analysing a sample using a dispersed structured illumination field.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is providedan apparatus for analysing a sample, said apparatus comprising aconfocal microscopy system comprising:

-   -   a first optical system comprising one or more optical sources        for emitting light in a first wavelength band and a wavelength        dispersive element for generating, at a first region of a        sample, a dispersed structured illumination field in the form of        a grid of beamlets for each wavelength within said first        wavelength band;    -   a second optical system for collecting light from said dispersed        structured illumination field reflected or scattered from said        first region of said sample, compensating for the spectral        dispersion imposed by said wavelength dispersive element and        passing the dispersion compensated collected light through at        least one aperture; and    -   a spectrometer comprising a two-dimensional sensor array for        spectrally analysing the collected light reflected or scattered        from said first region of said sample.

In certain embodiments the confocal microscopy system is configured toanalyse a structure in the posterior segment of an eye such that, inuse, the optical power elements of the eye cooperate with the firstoptical system to generate the dispersed structured illumination fieldat the structure. In a preferred embodiment the confocal microscopysystem is configured to analyse the retina of an eye.

In certain embodiments the first optical system is configured todisperse the light in the first wavelength band along a direction at anangle to an axis of a grid of beamlets, for generating a dispersedstructured illumination field that is substantially contiguous over thefirst region.

In certain embodiments the apparatus comprises an optical coherencetomography (OCT) system for performing an OCT analysis of the sample ata second region that is at least partially overlapping with the firstregion. In preferred embodiments the OCT system utilises light in asecond wavelength band, different from the first wavelength band, fordispersion onto different portions of the two-dimensional sensor array.Preferably, the OCT system comprises a light source for emitting lightin the second wavelength band. In alternative embodiments the confocalmicroscopy system and the OCT system are configured to analyse thesample utilising light in different polarisation states.

The spectrometer is preferably common to the OCT system and the confocalmicroscopy system. Preferably, the OCT system is configured to sample inthe Fourier field light reflected or scattered from the second region ofthe sample. Preferably, the OCT system comprises an aperture forfiltering out light that is outside the spatial Nyquist limit of ananalysis system of the OCT system.

In preferred embodiments the apparatus is configured to move thedispersed structured illumination field and a sample beam of the OCTsystem relative to the sample, for acquiring a plurality of confocalmicroscopy and OCT images of successive first and second regions of thesample. The apparatus is preferably configured to acquire, for eachposition of the dispersed structured illumination field and the samplebeam on the sample, a confocal microscopy image and an OCT image in asingle frame of the two-dimensional sensor array. Preferably, theapparatus comprises a processor adapted to register the plurality of OCTimages using information obtained from the plurality of confocalmicroscopy images. In preferred embodiments each of the second regionsis wholly within a respective first region.

Preferably, the apparatus comprises an optical splitter for splittingoff a portion of light from the one or more optical sources to form areference beam for interfering with light in the first wavelength bandreflected or scattered from the sample.

In certain embodiments the first optical system is configured todisperse the light in the first wavelength band along a directionsubstantially parallel to an axis of a grid of beamlets, for generatinga dispersed structured illumination field comprising one or moreoverlapping sequences of dispersed wavelengths spaced apart on thesample.

In certain embodiments the wavelength dispersive element has opticalpower for enabling wavelength-dependent focusing of the dispersedstructured illumination field. Preferably, the apparatus comprises aprocessor adapted to use the overlapping of the sequences of dispersedwavelengths at one or more points on the sample to provide a subsampledoptical spectrum for interferometric detellnination of an axial positionat the one or more points. The apparatus is preferably configured tomove the dispersed structured illumination field relative to the sample,for analysing additional regions of the sample.

In certain embodiments the first optical system comprises an opticalsource and a lenslet array for producing a grid of beamlets containinglight in the first wavelength band. In other embodiments the firstoptical system comprises a plurality of optical sources for producing agrid of beamlets containing light in the first wavelength band.

According to a second aspect of the present invention there is provideda method for analysing a sample, said method comprising the steps of:

-   -   generating at a first region of said sample, using one or more        optical sources emitting light in a first wavelength band and a        wavelength dispersive element, a dispersed structured        illumination field in the form of a grid of beamlets for each        wavelength within said first wavelength band;    -   collecting light from said dispersed structured illumination        field reflected or scattered from said first region of said        sample;    -   compensating for the spectral dispersion imposed by said        wavelength dispersive element;    -   passing the dispersion compensated collected light through at        least one aperture; and    -   spectrally analysing the collected light reflected or scattered        from said first region of said sample using a spectrometer        comprising a two-dimensional sensor array.

In certain embodiments the sample comprises a structure in the posteriorsegment of an eye. Preferably, the sample comprises the retina of aneye.

In preferred embodiments the method further comprises the step ofperforming an OCT analysis of the sample at a second region that is atleast partially overlapping with the first region. Preferably, themethod further comprises the steps of moving the dispersed structuredillumination field and a sample beam used for the OCT analysis relativeto the sample and acquiring a plurality of confocal microscopy and OCTimages of successive first and second regions of the sample. Preferably,for each position of the dispersed structured illumination field and thesample beam on the sample, a confocal microscopy image and an OCT imageare acquired in a single frame of the two-dimensional sensor array. Inpreferred embodiments the method further comprises the step ofregistering the plurality of OCT images using information obtained fromthe plurality of confocal microscopy images.

According to a third aspect of the present invention there is providedan apparatus for performing optical coherence tomography (OCT) imagingacross an extended region of a sample, said apparatus comprising:

-   -   a first optical system comprising one or more optical sources        for emitting light in a first wavelength band and a wavelength        dispersive element for generating, at a first region of a        sample, a dispersed structured illumination field in the form of        a grid of beamlets for each wavelength within said first        wavelength band;    -   a second optical system for collecting light from said dispersed        structured illumination field reflected or scattered from said        first region of said sample and compensating for the spectral        dispersion imposed by said wavelength dispersive element;    -   a spectrometer comprising a two-dimensional sensor array for        spectrally analysing the collected light reflected or scattered        from said first region of said sample;    -   an OCT system for acquiring an OCT image of said sample at a        second region that is at least partially overlapping with said        first region;    -   a mechanism for moving said dispersed structured illumination        field and a sample beam of said OCT system across said sample so        as to collect light reflected or scattered from at least one        additional first region and to acquire an OCT image from at        least one additional second region; and    -   a processor adapted to use information obtained from the        spectral analysis of the collected light from two or more first        regions to register OCT images acquired from two or more second        regions.

According to a fourth aspect of the present invention there is provideda method for performing optical coherence tomography (OCT) imagingacross an extended area of a sample, said method comprising the stepsof:

-   -   (i) generating at a first region of said sample, using one or        more optical sources emitting light in a first wavelength band        and a wavelength dispersive element, a dispersed structured        illumination field in the form of a grid of beamlets for each        wavelength within said first wavelength band;    -   (ii) collecting light from said dispersed structured        illumination field reflected or scattered from said first        region;    -   (iii) compensating for the spectral dispersion imposed by said        wavelength dispersive element;    -   (iv) spectrally analysing the collected light reflected or        scattered from said first region;    -   (v) obtaining, with an OCT system having a sample beam that        illuminates said sample, an OCT image of a second region of said        sample, wherein said second region is at least partially        overlapping with said first region;    -   (vi) moving said dispersed structured illumination field and        said sample beam relative to said sample and repeating steps (i)        to (v) for at least one additional first region and at least one        additional second region; and    -   (vii) using information obtained from the spectral analysis of        the collected light from two or more first regions to register        the OCT images acquired from two or more second regions.

According to a fifth aspect of the present invention there is provided aspectrometer for analysing the spectra of a plurality of polarised lightbeams, said spectrometer comprising:

-   -   a polarisation beam splitter for directing optical power        according to polarisation state;    -   a wavelength dispersive element for dispersing a plurality of        polarised light beams;    -   a polarisation transformation system for transforming the        polarisation of said plurality of polarised light beams; and    -   a two-dimensional sensor array for recording the spectra of said        plurality of polarised light beams, wherein said spectrometer is        configured such that, in use, said polarisation beam splitter        directs incoming polarised light beams to said wavelength        dispersive element and directs the dispersed polarised light        beams to said two-dimensional sensor array.

Preferably, the wavelength dispersive element comprises a grating. Inpreferred embodiments the polarisation transformation system comprises amirror and a quarter wave plate configured such that, in use, theplurality of polarised light beams traverse the quarter wave platebefore and after being reflected from the mirror. The spectrometerpreferably comprises a focusing element for imaging the spectralcomponents of the dispersed polarised light beams onto thetwo-dimensional sensor array. In certain embodiments the spectrometercomprises a polariser for analysing the polarisation of the plurality ofpolarised light beams before the polarised light beams traverse thepolarisation beam splitter. Preferably, the wavelength dispersiveelement is oriented such that each light beam in the plurality ofpolarised light beams is dispersed onto a separate set of pixels of thetwo-dimensional sensor array.

According to a sixth aspect of the present invention there is providedan article of manufacture comprising a computer usable medium having acomputer readable program code configured to operate the apparatusaccording to the first or third aspect, or to implement the methodaccording to the second or fourth aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 illustrates in schematic form an apparatus comprising integratedconfocal microscopy and spectral domain OCT systems for examining theretina of an eye;

FIG. 2A shows in schematic form a preferred illumination pattern on asample retina formed by light from the integrated confocal microscopyand spectral domain OCT systems shown in FIG. 1;

FIG. 2B shows in schematic form another illumination pattern formed bylight from the integrated confocal microscopy and spectral domain OCTsystems shown in FIG. 1;

FIG. 3 shows in schematic form a surface metrology apparatus;

FIG. 4 depicts a spaced-apart set of overlapping wavelength-dispersedlines produced by the FIG. 3 apparatus, for interaction with a sample;and

FIG. 5 shows a mapping of a 2-D grid of beamlets dispersed onto a 2-Dsensor array.

DETAILED DESCRIPTION OF THE INVENTION

As mentioned in the Background section, OCT-based techniques foracquiring single snapshot 3-D images of relatively small volumes of asample such as a retina, typically with illuminated areas of order 100μm×100 μm, have been disclosed in Anderson et al Proc SPIE 9697 (8 Mar.2016) http://dx.doi.org/10.1117/12.2214801 and published US patentapplication No 2016/0345820 A1. Larger volumes can in principle beimaged by stitching together images of adjacent or slightly overlappingvolumes, but this can be adversely affected by sample movementespecially for in vivo samples such as the human eye. To overcome thislimitation we present a registration and viewing technique that providessnapshot measurements with spatial resolution comparable to that offeredby confocal microscopy systems such as scanning laser ophthalmoscopes(SLOs), but without requiring high speed laser sampling which can haveits own motion artefacts.

FIG. 1 shows in schematic form an apparatus 100 comprising integratedconfocal microscopy and spectral domain OCT systems for examining one ormore structures in the posterior segment of an eye 118. The apparatus isparticularly suitable for examining the retina 116 and will be describedwith reference to this application, but it is also suitable forexamining other posterior segment structures such as the choroid. Inthis example embodiment we employ wavelength multiplexing for the twosystems, with first and second optical sources 102, 104 emitting lightin first and second wavelength bands that are different from each otherfor dispersion onto different portions of a 2-D sensor array 174. In analternative embodiment different wavelength bands are obtained from asingle optical source using wavelength-selective optics such as anoptical fibre or bulk optics dichroic splitter. Wavelength multiplexingenables confocal microscopy and OCT data to be acquired simultaneously,which is preferred for reliable compensation of ocular motion asdescribed below. In preferred embodiments each of the first and secondwavelength bands comprises a single range of wavelengths, while in otherembodiments they are interleaved. In other embodiments not described indetail herein, orthogonal polarisation states or fast time multiplexingcould be used instead of wavelength multiplexing for the confocalmicroscopy and OCT systems. For example polarisation demultiplexingcould be used to separate an illumination beam into two polarisationstates and encode the angularly dispersed confocal microscopy portion ofthe light into one polarisation state before a polarisation recombiningelement recombines the light which is subsequently directed to thesample. In this embodiment, not further described herein, the detectionsystem resolves both polarisation states in the reflected light toenable an integrated confocal microscopy and OCT system.

In preferred embodiments the optical sources emit light in the near IRspectral region, however in general light in the IR, visible or UVspectral regions could be used depending on the application.

We first describe the operation of a high-resolution spectral domain OCTimaging system operating with light of wavelength around 870 nm. In thesystem as described, an OCT image of a retina 116 is obtained using a‘holoscopic’ technique in which the Fourier plane of the image iscaptured and a fast Fourier transform (FFT) employed to simulatemathematically the role of a physical lens to construct ahigh-resolution image of the retina. Near field techniques canalternatively be used. Light from a second optical source 104 in theform of an optical fibre coupled superluminescent diode (SLD) withcentre wavelength 870 nm and bandwidth of approximately 4 nm is split bya 90/10 ratio 2×2 coupler 106 into a sample path 108 (90%) and areference path 110 (10%). Path length matching conditions will generallybe required for the interference portion of the OCT system, and can beachieved by adjusting the lengths of the respective optical fibre pathsduring manufacture or by providing length adjusting means such asswitchable delay lines for example. Light from the sample path fibre 108is collimated by a lens 112 to form a relatively small diameter samplebeam 114 of, say, several 100 μm FWHM, corresponding inverselyproportionally to the desired illumination spot size on the retina 116of a sample eye 118. That is, a smaller diameter sample beam willproduce a larger illumination spot size on the retina. A dichroic opticin the form of a dichroic beam splitter 120 designed to pass light of870±18 nm and reflect light of 840±8 nm allows the sample beam 114 to becombined with light 122 from a first optical source 102 to be describedlater. Other dichroic beam splitters 138, 148 and 156 in the apparatusare of similar design to the dichroic beam splitter 120.

The sample beam 114 passes through a polarisation beam splitter (PBS)124 and a quarter wave plate 126 before being relayed to the sample eye118 via a 4F lens system comprising lenses 128 and 130. In certainembodiments the polarisation of light from the second optical source 104is controlled, e.g. with a fibre optic polarisation controller or alinear polariser (not shown), to maximise its transmission through thePBS 124 towards the sample eye 118. Adjustment of the position and angleof light incident on the retina 116 can be achieved via beam steeringoptics such as angular deflection devices 132 and 134, which forconvenience are depicted here as transmissive devices such as prismsalthough non-dispersive reflective devices such as angularly adjustablemirrors are generally to be preferred. The first deflection device 132allows the sample beam 114, and the corresponding reflected beam thatreturns along the same path, to be moved to different positions on theretina 116. The second deflection device 134 allows the angle ofillumination on the retina to be adjusted, which can be important inachieving improved image quality through reduction of speckle andenhancement of resolving power. Additional power elements may beinserted into the relay to accommodate for variations of the eye undertest as would be well understood in the art.

A portion of the light 136 from the sample beam 114 reflected orscattered from the retina 116 is collected and passes back through therelay. The returning light then traverses the quarter wave plate 126 asecond time so that a significant fraction of it is polarisedorthogonally to the incoming sample beam 114, for direction by the PBS124 through a dichroic beam splitter 138 towards an optional holoscopyaperture 140 located at a position that, in use, corresponds to the 8Fretinal plane as defined by the combination of lenses 142, 128, 130 andthe optical power elements of the eye 118, i.e. its lens 103 and cornea101. Although optional, this aperture 140 advantageously serves tofilter out light that is outside the spatial Nyquist limit of thesubsequent analysis system and might otherwise contribute to phantomsignals or noise. Although the holoscopy aperture 140 is shown as beinglocated at the 8F retinal plane, it could alternatively be positionedelsewhere, for example at the 12F retinal plane.

The relayed retinal image is then collimated by a lens 144 after beingreflected by a mirror 146 and transmitted through a dichroic beamsplitter 148. A beam splitter 150, preferably but not necessarily apolarisation beam splitter (PBS), allows the far field of the retinalimage to be combined with a suitably path length adjusted reference beam152 that is collimated by a lens 154 and passes through a dichroic beamsplitter 156. The far field of the retinal image is then compared to thereference beam 152 using a spectrometer 158 capable of analysing thepolarisation state of the combined beams at a grid of spatial positionsdetermined by a spatial sampling element such as a two-dimensional (2-D)lenslet array 160, preferably in combination with a corresponding 2-Daperture array 162 for reducing stray light. The polarisation state ofthe combined beams is analysed by a PBS 164 which is adjusted to be atan angle to both polarisation states, thereby creating an interferencebetween the reference and signal paths which can provide information onthe relative phase across the far field of the retinal image. Thespectrometer 158 used here is a compact reflective spectrometer able toanalyse a plurality of grid points, beams or beamlets simultaneously asthey are dispersed at an angle to the grid by an appropriately orientedwavelength dispersive element in the form of a transmissive grating 166.A focusing element such as a lens 168 or an off-axis parabolic mirrorcollimates the grid of points for dispersion by the grating 166,followed by double passage through a quarter wave plate 170 viareflection from a mirror 172 to rotate the polarisation state by 90degrees. In combination the quarter wave plate 170 and the mirror 172form a polarisation transformation system, which in this particularexample effects a polarisation transformation comprising a 90 degreerotation. The dispersed spectral components of the reflected light areimaged by the lens 168 onto a 2-D sensor array 174 such as a CMOS cameraafter passing through the PBS 164. The interferogram detected by the 2-Dsensor array is read out in a single frame for subsequent analysis by aprocessor 176 equipped with suitable machine-readable program code. Theprocessor may for example apply well-known Fourier transform techniquesto obtain a depth-resolved image, i.e. a three-dimensional (3-D) imageof a volume corresponding to the area of the retina 116 illuminated bythe sample beam 114. In preferred embodiments the grating 166 isoriented with respect to the grid of spatial positions determined by the2-D lenslet array 160 and the corresponding 2-D aperture array 162 suchthat each of the combined beams entering the spectrometer 158 isdispersed onto a separate set of pixels of the two-dimensional sensorarray 174.

In broad terms the spectrometer 158 is configured for dispersion andpolarisation transformation of light at a grid of sampling points, wherethe dispersion allows analysis of different wavelengths and thepolarisation transformation enables redirection of optical power at apolarisation beam splitter. Advantageously, it provides a low-loss andcompact realisation of a grid-sampling spectrometer where thepolarisation beam splitter can provide the additional function of theinterference analyser.

We now describe the operation of a confocal microscopy system usingdispersed structured illumination, and its integration with theabove-described holoscopic spectral domain OCT imaging system. In theparticular embodiment described with reference to FIG. 1 the confocalmicroscopy system utilises light 122 from a first optical source 102 inthe form of an optical fibre coupled SLD with centre wavelength 840 nmand bandwidth of approximately 16 nm. Optionally, a portion e.g. 10% ofthe light from this source is split off with a 2×2 coupler 177 into areference path 178 if interferometric measurement is desired, e.g. forhigher sensitivity or to obtain relative phase measurements for examplefor angiographic phase contrast measurements. This optional referencepath 178 may also include an optical switch 179 to block the referencebeam for non-interferometric measurements, as well as path lengthadjustment means such as switchable delay lines for interferometricmeasurements.

Light 122 from the first source 102 is collimated by a lens 180 into arelatively large beam, e.g. several mm in diameter and preferably largerin diameter than the OCT sample beam 114, then launched into a spatialsampling element such as a 2-D lenslet array 181 and Fourier transformedby a subsequent lens 182. The Fourier transformed sample beamlets aredispersed by a wavelength dispersive element in the form of atransmissive grating 184 and coupled into the same train as theholoscopic OCT system by the dichroic beam splitter 120. Importantly,the lenslet array 181 and the grating 184 are preferably angled withrespect to each other so that, as described below, the grid of dispersedsample beamlets 183 will provide a dispersed structured illuminationfield that covers an extended and preferably substantially contiguousregion when reimaged onto the retina 116 of the sample eye 118. The gridof wavelength-dispersed Fourier transformed sample beamlets, representedschematically by the envelope 183, is relayed to the eye 118,analogously to the OCT sample beam 114, by the optical relay systemcomprising lenses 128, 130 and deflectors 132, 134. The optical powerelements of the eye then act on the dispersed sample beamlets togenerate at the retina 116, as shown in FIG. 2A, a dispersed structuredillumination field 200 in the form of a grid of beamlets for eachwavelength within the first wavelength band, i.e. within the wavelengthband emitted by the first source 102. Representative grids of beamletsfor two particular wavelengths within the dispersion envelope 202 of thefirst wavelength band are indicated by the arrays of points 204 and 206.When the lenslet array 181 and the grating 184 are suitably angled withrespect to each other as noted above, the dispersed structuredillumination field 200 can be made substantially contiguous over a firstregion 208 on the retina. That is, within this first region there arefew or no parts of the retina not illuminated by the dispersedstructured illumination field. Each combination of grid point pluswavelength defines a specific location 210 on the retina, with someredundancy since the dispersion envelopes 202 are partially overlappingat least within a portion of the first region 208. The sample beam 114of the OCT imaging system illuminates a second region 212 of the retinathat is at least partially overlapping with the first region 208.Preferably the second region 212 is wholly within a larger first region208 as shown, so that every point on the retina illuminated by the OCTbeam 114 is also illuminated by the dispersed structured illuminationfield 200 of the confocal microscopy system. In certain embodiments thefirst region 208 may be ten to several hundred times larger than thecumulative area of the second region 212. The second region may becontiguous as shown, or a grid of sampling points (A-scans), generatedfor example by a lenslet array or other spatial sampling element in theOCT optics train (not shown in FIG. 1), each of a given area, providinga cumulative area. Returning to FIG. 1, in certain embodiments someoptical power can be provided by appropriate design of the grating 184,e.g. a degree of chirp, which would allow the redundant wavelengths tobe imaged at different depths to give greater tolerance to variations inthe ocular power of the eye 118 being analysed.

When the apparatus 100 is being used to analyse the retina 116 an eye118 as shown in FIG. 1, the optical power elements of the eye, i.e. thecornea 101 and the lens 103, cooperate with the lenslet array 181, theFourier transforming lens 182, the grating 184 and other opticalcomponents in the train to generate the dispersed structuredillumination field at the retina 116. For analysing a non-ocular samplethe apparatus 100 may include an additional focusing element in lieu ofthe optical power elements of the eye. Similarly, for analysing astructure in the anterior segment of an eye, such as the sclera, theanterior or posterior corneal surface or the anterior or posterior lenssurface, the apparatus 100 may include an additional focusing element inlieu of some or all of the optical power elements of the eye asappropriate. These additional focusing elements would also serve tofocus the OCT sample beam 114 onto the sample.

As opposed to the 3-D holoscopic measurement provided by the OCT imagingsystem, the dispersed structured illumination field 200 as shown in FIG.2A enables a confocal measurement at each illuminated point 210 on theretina 116. However to obtain the confocal measurement we need to createa pinhole or aperture array that can act on all wavelengthssimultaneously. This is provided by a second optical system comprising adispersive element in the form of a transmissive grating 185 forunwinding the dispersion imposed by the grating 184, and an aperturearray 186 that is preferably tightly correlated with the sampling gridthat was created by the lenslet array 181. More broadly, the aperturearray 186 needs to have at least one aperture correlated tightly with atleast part of the sampling grid generated by the lenslet array 181. Thedispersion compensation from the grating 185 occurs after the confocalbeamlets 187 collected from the retina 116 have been separated from thereturning holoscopic OCT beam 105 by the dichroic beam splitter 138. Thelens 188 used to focus the confocal beamlets 187 onto the aperture array186 may require active focal length adjustment in production to ensureregistration at the aperture array 186 over the whole wavelength rangeand grid points. The second grating 185 also needs to be designed toreverse any optical power that may have been designed into the firstgrating 184. An optional lenslet array 189 can be used to optimise theprojection of the aperture array 186 through the lenslet array 160 andonto the subsequent aperture array 162 with low coupling loss, althoughadjustments of the lenses 190 and 144 may prove adequate in practicewith reasonable efficiency. The grid of dispersion compensated beamlets191 is recombined with the returning OCT beam 105 in the dichroic beamsplitter 148, for analysis by the spectrometer 158. This spectrometer iscapable of analysing the grid of confocal microscopy points and thelenslet array 160-sampled returning OCT beam 105 over a range of 830 to890 nm through design of the dispersive element 166 and the focusinglens 168. It will be appreciated that because the confocal microscopyand OCT systems use light in different wavelength bands, the grid ofconfocal microscopy points and the sampled returning OCT beam will beprojected onto different portions of the 2-D sensor array 174, enablingacquisition of both confocal microscopy and OCT data within a singleframe of the 2-D sensor array.

As mentioned above and with reference to FIG. 2A, adjustment of theangular deflection device 132 enables the region 212 illuminated by theOCT sample beam 114 to be moved to different positions on the retina116, enabling examination of larger areas. Since the OCT and confocalmicroscopy systems are integrated, this adjustment also moves the region208 illuminated by the dispersed structured illumination field 200 ofthe confocal microscopy system. Importantly, because the region 212 isat least partially overlapping with, and preferably wholly within alarger region 208, the confocal microscopy system can be utilised toensure accurate registration of successive volumes illuminated by theOCT sample beam 114. For example the processor 176 may useautocorrelation techniques to determine displacement vectors based onthe overlapping images of the larger region 208. If eye movement, say,causes an error in the positioning of the OCT sample beam 114 on theretina, this error can be detected and corrected by matching the largerarea retinal images acquired by the confocal microscopy system. It isfor the purpose of ensuring accurate registration of successive OCTvolumes that the dispersed structured illumination field 200 ispreferably angled with respect to the beamlet grids 204, 206 asdescribed above with reference to FIG. 2A. If the dispersion axis of thegrating 184 were parallel to an axis of the lenslet array 181 thedispersed structured illumination field 200 at the retina may appear asshown in FIG. 2B. It will be appreciated that in this situation thereare gaps 214 in the dispersed structured illumination field 200 wheresignificant portions of a region 212 illuminated by the OCT sample beamwould have no overlap with the dispersion envelopes 202.

Within each frame of the 2-D sensor array 174 the actual illuminationtime can be quite brief, of the order of 100 μs, to reduce any fringefading due to motion artefacts. For maximum accuracy of registration, agrid of confocal microscopy points and a lenslet-sampled OCT beam arepreferably acquired substantially simultaneously e.g. with respectiveOCT and confocal microscopy images acquired in a single frame of the 2-Dsensor array 174. It is also possible for confocal microscopy and OCTimages to be acquired in successive frames, e.g. by shuttering of thedifferent wavelength bands obtained from a single broadband source or bypulsing of separate optical sources, although the time between framesshould be short so that ocular motion can be reliably compensated. Theability to choose independently the timing of the illumination for theconfocal microscopy and OCT systems can be beneficial. In one example,pulsing the confocal microscopy system source 102 at the end of a frameprovides the ability to adjust the exact registration of the next frameof the holoscopic OCT measurement through a fine adjustment of thedeflection device 132 from the position anticipated based on the shiftof the confocal microscopy measurement. The ability to measure andcorrelate across a large field of the retina 116 allows us to ensurethat there are no gaps or stitching errors between separate volumesinduced by eye motion. Furthermore we can accurately return to anyretinal areas if required for reacquisition. Because the region 208illuminated by the confocal microscopy system is usually considerablylarger than the region 212 illuminated by the OCT system, points on theretina will often appear in several confocal images, possibly hundredsof images. Advantageously, these multiple exposures enable a highlyaveraged image to be generated by having measured each point multipletimes with different illumination wavelengths, increasing the quality ofthe image and reducing any speckle artefacts.

The confocal microscopy system can be operated in either a purelyintensity mode, i.e. without employing the reference arm 178, as wouldbe the case for a conventional SLO or scanning confocal microscope, orin a coherent mode where the grid of beamlets 191 reflected from theretina 116 is interfered with a reference beam 192 collimated by a lens193. Noting that a coherent signal can be more easily interpreted if itsintensity is known, acquisition in both modes in sequential frames ispossible via operation of the optical switch 179. In many cases thepurely intensity mode will be preferred, since the interference in thecoherent mode can be an unnecessary complication for the registrationprocess. Alternatively it is possible to acquire a phase-resolvedinterference spectrum to interpret or determine the relative phase andamplitude of the confocal microscopy signal.

It will be appreciated that several elements in the apparatus of FIG. 1may need to be operated in coordinated fashion, including for examplethe optical sources 102 and 104, the angular deflection devices 132 and134, the optical switch 179 and the 2-D sensor array 174. This overalllevel of control may for example be provided by equipping the processor176 with suitable machine-readable program code.

In preferred embodiments the spectrometer 158 is common to theintegrated OCT and confocal microscopy systems as shown in FIG. 1, toincrease integration of the componentry and ensure enhanced registrationof the OCT images. However it would be advantageous in somecircumstances to have two independent spectrometers, e.g. to enableindividual optimisation of each spectrometer.

We note that, for the purposes of registration of successive volumesilluminated by the OCT system, it is not essential for the integratedmicroscopy system to be confocal. That is, the aperture array 186 andthe lenslet array 189 could be omitted from the apparatus 100 to yieldan OCT imaging system integrated with a non-confocal microscopy system.However a confocal microscopy system is preferred because of theimproved noise rejection and the improved accuracy of registration inthe presence of axial displacements.

It will be appreciated that the confocal microscopy and OCT systems ofthe apparatus 100 can be operated in isolation. For example the confocalmicroscopy system alone may be used to analyse one or more regions ofthe retina 116 or other structure in the posterior segment of an eye118.

The ability to generate a dispersed structured illumination field in theform of a grid of beamlets for each wavelength within a wavelength band,used for registration of repeated snapshot 3-D OCT images as describedabove, can also be used for metrology applications. FIG. 3 shows inschematic form an apparatus 300 for surface metrology of an object 320.Light from an optical source 302 in the form of a polarisedsuperluminescent light emitting diode (SLED) with centre wavelength 840nm and a bandwidth of approximately 20 nm is collimated by an off-axisparabolic mirror 304 then passed through a spatial sampling element suchas a 2-D lenslet array 310 or an array of diffractive optical elements(DOEs) to produce a grid of sample beamlets. The sample beamlets arerelayed through a system of lenses 311 and 312 which can be magnifyingor reducing, depending on the application, through choice of focallength. A wavelength dispersive element in the form of a transmissivegrating 306 having, say, 1500 lines/mm, is provided within the lensrelay. The optical source 302, lenslet array 310, grating 306 and lenses311, 312 in combination form an optical system that generates adispersed structured illumination field 314 in the form of a grid ofbeamlets for each wavelength within the emission band of the source.That is, each wavelength component of the source 302 will have acorresponding grid which is offset in the dispersive axis of the grating306. In an alternative embodiment a grid of sample beamlets forgenerating the dispersed structured illumination field is provided by aplurality of optical sources such as an array of LEDs. The dispersedstructured illumination field 314 proceeds to the sample 320 via a beamsplitter, preferably but not necessarily a polarisation beam splitter(PBS) 322, that splits it into a sample arm 324 and a reference arm 326,and the reflections from the sample 320 and the reference arm mirror 328are recombined by the PBS 322 after rotation of their respectivepolarisations by the quarter wave plates 330 and 332.

In certain embodiments the transmissive grating 306 is designed toprovide optical power in one or two axes through curvature of the linesor a chirped line spacing, or both, as is known in the art. In this casethe optical power of the grating is a function of wavelength, so thatany subsequent focusing of the light will have an effectivewavelength-dependent focal plane as represented by the curved wavefronts309. This feature is particularly advantageous for an industrialmetrology system where samples may have large surface fluctuations, asit permits high transverse resolution over an extended depth of focus byusing the chromatic focusing in combination with the phase informationof the light at multiple wavelengths, which can be accessed through aninterferometric OCT measurement described below.

In the metrology apparatus 300 the grating 306 and lenslet array 310 arepreferably oriented with respect to each other such that the dispersiveaxis of the grating is aligned with an axis of the lenslet array. Thisis in contrast to the previously described confocal microscopy/OCTapparatus 100 shown in FIG. 1, where the dispersive axis of the grating184 is preferably at an angle to an axis of the lenslet array 181. Asdepicted schematically in FIG. 4, dispersion along an axis of thelenslet array results in a dispersed structured illumination field 400having overlapping sequences of dispersed wavelengths 402 spaced aparton a series of projection lines 404, for interaction with a sample.Similar to the situations shown in FIGS. 2A and 2B, the arrays of points406 and 408 represent grids of beamlets for two particular wavelengthswithin the dispersion envelope 410 of the wavelength band emitted by thesource 302.

The combined reflected reference and sample light fields are transformedthrough a dispersive optical relay 334 comprising relay lenses 336 and338 and a numerical aperture limiting grating 340 designed to compensatethe spectral dispersion and any chromatic focusing imposed by thegrating 306. This enables all wavelengths in the grid of dispersioncompensated beamlets 341 to be simultaneously apertured by an aperturegrid 342 designed to correspond to the illuminating grid produced by thelenslet array 310. The size of the apertures in the aperture grid 342and the size of the dispersive relay aperture defined by the grating 340are chosen to optimise collection of light from a sufficient depth offield at the sample 320 and reduce crosstalk from multiply-scatteredlight. Generally, smaller apertures provide enhanced depth of field andreduced lateral resolution, along with lower signal levels. In oneparticular example the combination of a 4 mm long grating 340 and a lens336 with 70 mm long focal length would provide a numerical aperture ofapproximately 0.02 depending on the angle of the grating 340. Anoptional lenslet array 344 can be used to adjust the numerical apertureof the incident beamlets 341 to match the aperture size and hence therequired resolution of the spectrometer 346. In certain embodiments anadjustable polariser 348 can be included to capture a fraction of thepower in the combined sample and reference beams according to theexpected power levels in each arm, and so interfere the sample andreference light according to their relative phases. Alternatively thepolarisation state of the combined beams can be analysed by thepolarisation beam splitter (PBS) 350, as is the case with thespectrometer 158 in the apparatus shown in FIG. 1. Inclusion of thepolariser 348 allows the polarisations to be analysed substantiallyindependently of the PBS 350.

The dispersion compensated grid of sampling points entering thespectrometer 346 contains sample information derived from reflection orscattering from the illuminated portions of the sample 320, and can beanalysed by 2-D spectrometer techniques as described in published USpatent application Nos US 2016/0135679 A1 entitled ‘Ocular metrologyemploying spectral wavefront analysis of reflected light’ and US2016/0135680 A1 entitled ‘Wavefront analyser’, the contents of which areincorporated herein by reference. In the example embodiment depicted inFIG. 3 a compact spectrometer 346 comprises a PBS 350, a flat fieldrelay lens 352 or other focusing element, a wavelength dispersiveelement in the form of a transmissive grating 356 and a polarisationtransformation system 354 comprising a quarter wave plate 358 and amirror 360. The polarisation transformation system rotates thepolarisation state of the dispersed light by 90 degrees to allowre-direction at the PBS 350 onto a 2-D sensor array 362 such as a CMOScamera for analysis by a processor 364 equipped with suitablemachine-readable program code. The grating 356 is preferably orientedsuch that the dispersion is at an angle to the sampling gridcorresponding to the aperture grid 342, to allow independentinterferometry of each resolvable wavelength for each point in thesampling grid. That is, each grid point 500 is dispersed onto a separateset of pixels 502 of a 2-D sensor array 504 as shown in FIG. 5.

The dimensions of the on-sample dispersed structured illumination field400 shown in FIG. 4 will depend on the details of the optical componentsthat generate it. In one exemplary embodiment each overlapping sequenceof dispersed wavelengths 402 has a length 412 of about 10 mm and a width414 of about 20 μm, on a centre-to-centre spacing 416 of about 200 μm,with the number of projection lines 404 determined by the number of rowsof lenslets in the lenslet array 310. Each point 418 on the samplewithin one of the overlapping sequences of dispersed wavelengths 402will be illuminated by multiple wavelengths, which enablesinterferometric techniques to determine an unambiguous measurement ofeach axial reflection point over a certain depth using a subsampledspectrum. In embodiments where the grating 306 has optical power, therelative power in the reflection points will be significantly determinedby the wavelength dependent focusing. This can provide furtherinformation on the reflection points to extend further the range overwhich an unambiguous determination of depth can be made. Additionally,continuity constraints can often be employed to enhance the relativeaccuracy of measurements.

We now consider the effect of scanning the dispersed structuredillumination field 400 in a controlled fashion relative to the sample320, e.g. by translating the sample on a stage or by using beam steeringoptics. After the 2-D sensor array 362 and the processor 364 haveacquired and analysed the sample volume corresponding to the spacedapart overlapping sequences of dispersed wavelengths 402 at one setposition, the acquisition can be repeated with steps of, say, 10 μmuntil the spaces 420 between the overlapping sequences are filled, toacquire a complete contiguous volume. A large jump can then be made ontoan adjacent sample region and the process repeated. Preferably the stepsare made at a small angle to the perpendicular of the projection lines404 to allow oversampling of each sample point 418 at a different set ofwavelengths. It is not essential for the dispersed structuredillumination field 400 to have a plurality of overlapping sequences ofdispersed wavelengths 402 as shown in FIG. 4, although this would beadvantageous for faster measurement of a sample over an extended area.On the other hand the use of a dispersed structured illumination fieldhaving only one overlapping sequence of dispersed wavelengths 402, whichcould for example be produced with a 1-D lenslet array or other 1-Dspatial sampling element, may be advantageous for more rapid datareadout.

In an alternative embodiment a wavelength-dispersed structuredillumination field is generated by interfering an array ofwavelength-dispersed beamlets generated for example with a 2-D lensletarray. As described in Besold et al ‘Fractional Talbot effect forperiodic microlens arrays’, Optical Engineering 36(4), 1099-1105 (1997),the coherent superposition of multiple dispersed beamlets forms a gridof high intensity ‘rods’ which can propagate with coherent superpositionover a distance much longer than the corresponding Rayleigh length of anequivalently sized array of individual beamlets.

Although the invention has been described with reference to specificexamples, it will be appreciated by those skilled in the art that theinvention may be embodied in many other forms.

What is claimed is:
 1. An apparatus for analysing a sample, saidapparatus comprising a confocal microscopy system comprising: a firstoptical system comprising one or more optical sources for emitting lightin a first wavelength band and a wavelength dispersive element forgenerating, at a first region of a sample, a dispersed structuredillumination field in the form of a grid of beamlets for each wavelengthwithin said first wavelength band; a second optical system forcollecting light from said dispersed structured illumination fieldreflected or scattered from said first region of said sample,compensating for the spectral dispersion imposed by said wavelengthdispersive element and passing the dispersion compensated collectedlight through at least one aperture; and a spectrometer comprising atwo-dimensional sensor array for spectrally analysing the collectedlight reflected or scattered from said first region of said sample. 2.The apparatus according to claim 1, wherein said confocal microscopysystem is configured to analyse a structure in the posterior segment ofan eye such that, in use, the optical power elements of said eyecooperate with said first optical system to generate said dispersedstructured illumination field at said structure.
 3. The apparatusaccording to claim 2, wherein said confocal microscopy system isconfigured to analyse the retina of an eye.
 4. The apparatus accordingto claim 1, wherein said first optical system is configured to dispersethe light in said first wavelength band along a direction at an angle toan axis of a grid of beamlets, for generating a dispersed structuredillumination field that is substantially contiguous over said firstregion.
 5. The apparatus according to claim 1, comprising an opticalcoherence tomography (OCT) system for performing an OCT analysis of saidsample at a second region that is at least partially overlapping withsaid first region.
 6. The apparatus according to claim 5, wherein saidOCT system utilises light in a second wavelength band, different fromsaid first wavelength band, for dispersion onto different portions ofsaid two-dimensional sensor array.
 7. The apparatus according to claim6, wherein said OCT system comprises a light source for emitting lightin said second wavelength band.
 8. The apparatus according to claim 5,wherein said confocal microscopy system and said OCT system areconfigured to analyse said sample utilising light in differentpolarisation states.
 9. The apparatus according to claim 5, wherein saidspectrometer is common to said OCT system and said confocal microscopysystem.
 10. The apparatus according to claim 5, wherein said OCT systemis configured to sample in the Fourier field light reflected orscattered from said second region of said sample.
 11. The apparatusaccording to claim 5, wherein said OCT system comprises an aperture forfiltering out light that is outside the spatial Nyquist limit of ananalysis system of said OCT system.
 12. The apparatus according to claim5, wherein said apparatus is configured to move said dispersedstructured illumination field and a sample beam of said OCT systemrelative to said sample, for acquiring a plurality of confocalmicroscopy and OCT images of successive first and second regions of saidsample.
 13. The apparatus according to claim 12, wherein said apparatusis configured to acquire, for each position of said dispersed structuredillumination field and said sample beam on said sample, a confocalmicroscopy image and an OCT image in a single frame of saidtwo-dimensional sensor array.
 14. The apparatus according to claim 12,wherein said apparatus comprises a processor adapted to register theplurality of OCT images using information obtained from the plurality ofconfocal microscopy images.
 15. The apparatus according to claim 12,wherein each of the second regions is wholly within a respective firstregion.
 16. The apparatus according to claim 1, comprising an opticalsplitter for splitting off a portion of light from said one or moreoptical sources to form a reference beam for interfering with light insaid first wavelength band reflected or scattered from said sample. 17.The apparatus according to claim 1, wherein said first optical system isconfigured to disperse the light in said first wavelength band along adirection substantially parallel to an axis of a grid of beamlets, forgenerating a dispersed structured illumination field comprising one ormore overlapping sequences of dispersed wavelengths spaced apart on saidsample.
 18. The apparatus according to claim 17, wherein said wavelengthdispersive element has optical power for enabling wavelength-dependentfocusing of said dispersed structured illumination field.
 19. Theapparatus according to claim 17, wherein said apparatus comprises aprocessor adapted to use the overlapping of the sequences of dispersedwavelengths at one or more points on said sample to provide a subsampledoptical spectrum for interferometric determination of an axial positionat said one or more points.
 20. The apparatus according to claim 17,wherein said apparatus is configured to move said dispersed structuredillumination field relative to said sample, for analysing additionalregions of said sample.
 21. The apparatus according to claim 1, whereinsaid first optical system comprises an optical source and a lensletarray for producing a grid of beamlets containing light in said firstwavelength band.
 22. The apparatus according to claim 1, wherein saidfirst optical system comprises a plurality of optical sources forproducing a grid of beamlets containing light in said first wavelengthband.
 23. An article of manufacture comprising a computer usable mediumhaving a computer readable program code configured to operate theapparatus of claim
 1. 24. A method for analysing a sample, said methodcomprising the steps of: generating at a first region of said sample,using one or more optical sources emitting light in a first wavelengthband and a wavelength dispersive element, a dispersed structuredillumination field in the form of a grid of beamlets for each wavelengthwithin said first wavelength band; collecting light from said dispersedstructured illumination field reflected or scattered from said firstregion of said sample; compensating for the spectral dispersion imposedby said wavelength dispersive element; passing the dispersioncompensated collected light through at least one aperture; andspectrally analysing the collected light reflected or scattered fromsaid first region of said sample using a spectrometer comprising atwo-dimensional sensor array.
 25. The method according to claim 24,wherein said sample comprises a structure in the posterior segment of aneye.
 26. The method according to claim 24, further comprising the stepof performing an OCT analysis of said sample at a second region that isat least partially overlapping with said first region.
 27. An article ofmanufacture comprising a computer usable medium having a computerreadable program code configured to implement the method of claim 24.28. An apparatus for performing optical coherence tomography (OCT)imaging across an extended region of a sample, said apparatuscomprising: a first optical system comprising one or more opticalsources for emitting light in a first wavelength band and a wavelengthdispersive element for generating, at a first region of a sample, adispersed structured illumination field in the form of a grid ofbeamlets for each wavelength within said first wavelength band; a secondoptical system for collecting light from said dispersed structuredillumination field reflected or scattered from said first region of saidsample and compensating for the spectral dispersion imposed by saidwavelength dispersive element; a spectrometer comprising atwo-dimensional sensor array for spectrally analysing the collectedlight reflected or scattered from said first region of said sample; anOCT system for acquiring an OCT image of said sample at a second regionthat is at least partially overlapping with said first region; amechanism for moving said dispersed structured illumination field and asample beam of said OCT system across said sample so as to collect lightreflected or scattered from at least one additional first region and toacquire an OCT image from at least one additional second region; and aprocessor adapted to use information obtained from the spectral analysisof the collected light from two or more first regions to register OCTimages acquired from two or more second regions.
 29. An article ofmanufacture comprising a computer usable medium having a computerreadable program code configured to operate the apparatus of claim 28.